1. Field of the Invention
This invention relates to an image pickup device having an improved arrangement of color separation filters.
2. Description of the Prior Art
In the conventional image pickup devices where, for example, a solid-state image scanner having 384 elements in the horizontal direction and 490 elements in the vertical direction thereof is applied to a monochrome camera, the horizontal read-out clock frequency fc of the solid-state image scanner corresponds to 2 fsc=7.16 MHz of a color sub-carrier frequency fsc with the frequency fsc 3.58 MHz. Since an image of an object to be photographed coming via a picture taking optical system such as a lens is to be sampled at the horizontal read-out clock frequency, horizontal resolution can be obtained to an extent corresponding to a Nyquist frequency fN (fc/2=3.58 MHz). This is thus sufficient for horizontal resolution in currently available TV receivers. If such an image scanner is provided for each of the colors including red (R), green (G) and blue (B) to form a three-plate type color camera, the camera would be capable of giving resolution and color reproducibility for high picture quality such as in a three-tube type color camera.
However, in a three-plate type color camera, it is difficult to reduce the size thereof because the structural arrangement and adjustment of a tri-color separation optical system thereof is complex. Another problem with this type of color camera is that cost reduction to a substantial degree is hardly possible. In view of these problems, it is necessary, for home use, to develop some single-plate type color camera that has a single solid-state image scanner. An attempt to develop a color camera of that type encounters unsolved problems. Consider a problem relative to resolution. A color camera, of course, requires three primary color signals including a red signal, a blue signal and a green signal. In the single-plate type color camera, therefore, the three primary colors must be suitably distributed. In distributing the three primary colors, color separation filters are used in accordance with known methods which can be roughly divided into a mosaic filter method, which is called the Bayer arrangement, and a stripe method in which filters of the same color are arranged in the vertical direction of the solid-state image scanner.
FIG. 1 of the accompanying drawings schematically illustrates a color separation filter of the R, G and Cy stripe type. When the stripe filter of R, G, Cy is used and the clock frequency fc is 7.16 MHz as mentioned above, a sampling frequency for each color in the horizontal direction becomes 1/3 fc=2.4 MHz. Then, the bandwidth color of the chrominance signal becomes about 1.2 MHz. Therefore, a luminance signal obtained from this chrominance signal would result in a horizontal resolution which is about 1/3 of a monochrome camera giving poor picture quality.
In order to obtain from the R, G and B stripe type camera, a horizontal resolution comparable with that of the three plate type, the number of horizontal elements of the image scanner must be greatly increased. However, it is extremely difficult to approximately triple the number of elements in the horizontal direction even with the current LSI technology. In view of this difficulty, Japanese Patent Laid-Open Application No. Sho 56-120281 (or No. 120281-81) has disclosed a method wherein the number of horizontal elements is increased by about 50% and up to about 570 with a color separation filter such as the filter shown in FIG. 1. This method is characterized in that the color separation filter is placed in a stripe type filter arrangement consisting of colors including red (R), green (G) and a cyan color Cy (Cy=B+G). The filter is arranged such that, when a picture of a white object is taken, the ratio of the level of each signal corresponding to a light passing through each of the filters including an R filter FR, a G filter FG and a Cy filter Fcy becomes 1:1:1. With the filter arranged in this manner, a luminance signal is obtained directly from each color or chrominance signal for improved horizontal resolution. However, this prior art method is based on the presumption that picture taking objects generally have a low degree of saturation.
Referring to FIG. 2(a), in the above prior art method, a low saturation object is considered to be about the same as a monochromatic object and, thus, a sampling frequency is equivalently considered to be fc=10.7 MHz even for a chrominance signal. It is thus assumed that the luminance signal band is attainable up to fnl=fc/2. However, this method has the shortcoming that a folding noise occurs if the saturation of the object is high.
When an image of a monochromatic object (for example, a red flower) is taken, assuming that the frequency spectrum distribution of a baseband signal is as represented by a curve A in FIG. 2(b) and that the signal is limited by a band filter LPF1 of FIG. 2(a), the sampling frequency becomes 1/3 fc because it is dependent solely upon the red signal in this case. Then, in the output signal of a solid-state scanner, there appears a primary sideband signal B, etc., at the baseband signal (the curve A) at 1/3 fc. However, since a low-pass filter LPF1, which is used for obtaining a luminance signal, is a wide band as shown in FIG. 2(b), a component obtained by multiplying the sideband portion, indicated by hatching in FIG. 2(b), by the curve of the filter LPF1 appears as a folding noise, which seriously degrades the picture quality.
The color reproducibility of the above method has the following problem: Referring to FIG. 3(a), the spectral sensitivity characteristic of a stripe filter, which is composed of filters FR, FG and Fcy as shown in FIG. 3(a), is limited by an infrared cut filter. The sensitivity of the filter for a cyan color light is limited by the spectral sensitivity characteristic CCD. Therefore, the overall spectral sensitivity distribution of the image pickup device must have uniform sensitivity distribution for all of the colors including red, green and a cyan color by presetting and adjusting the transmission factor of each of the color filters.
In an image pickup device of the above R, G and Cy type, a signal SB for a blue color is obtained by subtracting a signal SG, which corresponds to a light which has passed through the green filter FG, from a signal Scy, corresponding to a light which has passed through the filter Fcy of the cyan color. Therefore, the spectral sensitivity distribution of the cyan color filter must be a composition of the sensitivity distribution of the blue color filter and that of the green color filter, as shown in FIG. 3(b), while in overall sensitivity, it must be the same as red and green. Conventional cyan color filters have sensitivity distributions as shown in FIG. 3(a). Therefore, the blue signal SB, which is obtained through the above correlating process, is different from the signal which is normally obtainable from the blue color filter. This results in a serious deterioration of color reproduction.
Let us now consider the cementing precision of the image scanner and the color separation filter. There are different kinds of known image scanners including an interline type CCD (IL type), a frame transfer type CCD (FT type), an MOS type, etc. However, the FT type CCD is used in the following description:
FIG. 4 is an enlarged view showing a portion of the image pickup part (or a photoelectric transducer) of the FT type CCD. The details will be described later herein. In FIG. 4, channel stoppers 3 prevent electric charge between horizontal directional elements from dispersing. To this part is applied and cemented the color separation filter which is shown in FIG. 1. There is provided light shielding layers LS for preventing incident light from passing through. The color separation filter consists of filter parts Cy, R and G which allow, respectively, a Cy light, an R light and a G light to pass therethrough. The channel stoppers normally measures 2 to 3 .mu.m in width while the light shielding layers LS are the same width as the channel stoppers. The filter is cemented with the channel stoppers and the light shielding layers LS in superposition with each other. However, it is extremely difficult to increase the cementing precision because of mechanical error of cementing, the dimensional error of the image pickup element, that of the filter, etc. In addition, the color separation filter tends to be moved by adhesive's tension unevenness which arises during the stiffening process of an adhesive. If a cementing error causes, for example, the G filter part to overlap the image pickup element part of the R filter, a chrominance signal, which is to be ultimately obtained, would represent a mixed color which deteriorates color reproducibility.
Meanwhile, it is preferable to reduce the width of the light shielding layer LS of the color separation filter as much as possible. This is because the channel stopper has a certain degree of light sensitivity. Therefore, it is desired to increase sensitivity of the camera as a whole by increasing the (light receiving) aperture efficiency of the image pickup element through the reduction of the width of the light shielding layer LS.
As has been described in the foregoing, the conventional color separation filters have had many shortcomings in terms of resolution, color reproducibility, cementing precision, etc.